1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus which forms images by depositing a liquid such as ink onto a recording medium from an ejection head.
2. Description of the Related Art
An inkjet recording apparatus (inkjet printer) having an ink ejection head (print head) in which a large number of nozzles are arranged is known as a conventional image forming apparatus. This inkjet recording apparatus forms an image on a recording medium by ejecting ink in the form of liquid droplets from the nozzles of the ink ejection head onto the recording medium while moving the ink ejection head relatively to the recording medium.
Various conventional methods are known for ejecting the ink in this type of inkjet recording apparatus. Known examples of ink ejection methods include a piezoelectric method, in which a diaphragm constituting a part of a pressure chamber (ink chamber) is deformed by the deformation of a piezoelectric element (piezoelectric ceramic), thereby altering the volume of the pressure chamber such that when the pressure chamber volume increases, ink is introduced into the pressure chamber through an ink supply passage, and when the pressure chamber volume decreases, the ink inside the pressure chamber is ejected through the nozzles as an ink droplet, and a thermal inkjet method in which air bubbles are formed by heating the ink, and the ink is ejected by the expansion energy generated as the air bubbles grow.
In an image forming apparatus having an ink ejection head, such as an inkjet recording apparatus, ink is supplied to the ink ejection head from an ink tank which stores the ink via an ink supply passage, and the ink is ejected using one of the various ejection methods described above. When the ink used in such an image forming apparatus is deposited on the recording medium, it is desirable that the ink dry and become fixed immediately.
The nozzles of the ink ejection head are filled with ink at all times so that when a printing command is issued, printing can be executed immediately, but if the ink in the nozzles dries, ink ejection from the nozzles becomes unstable, and hence during standby periods when printing is not performed, the ink ejection head is sealed tightly by a cap to prevent the ink in the nozzles from drying.
However, during printing the ink in the nozzles is exposed to the air, and hence the ink in a nozzle from which ejection is not performed for a long time dries, leading to an increase in the viscosity (thickening) of the ink. As a result, the nozzle may become blocked, the ink in the nozzle may disappear, and hence ejection may become impossible.
Further, when bubbles that have become mixed into the ink supply passage and so on accumulate in front of a foreign matter removal filter disposed in the ink ejection head or ink supply passage, the accumulated bubbles may block the ink supply so that the ink cannot be ejected from the nozzles.
Conventionally, a purging (spitting) operation is performed at fixed time intervals to remove the viscous ink or ink containing foreign matter or bubbles that is the cause of the defective ejection and thereby restore the ink ejection head. In this purging operation, the cap is placed over the ink ejection head, and the ink is ejected forcibly from the nozzles toward the cap. Alternatively, a suction operation is performed periodically by placing the cap over the ink ejection head and suctioning the ink out of the nozzles using a pump.
If the purged or suctioned ink is discarded at this time, this constitutes wasteful ink consumption, and therefore, to limit the amount of wasted ink as much as possible, ink circulation is performed to return the suctioned ink to the ink tank (sub-tank) so that the ink can be re-supplied to the ink ejection head and reused.
For example, a device is known in which a head is sealed tightly with a cap, and a pump is driven to suction ink from the head into the cap. The ink suctioned into the cap is then led through a pipe to the sub-tank side, and returned to the sub-tank through a filter. Thus the waste ink is circulated and reused (see Japanese Patent Application Publication No. 2003-266745, for example).
However, in the conventional inkjet printer having an ink circulation path described in Japanese Patent Application Publication No. 2003-266745, the ink circulation path for returning the ink suctioned into the cap to the sub-tank is wound around the outside of the head, and therefore the ink circulation path is long and complicated, disposal space must be secured for the ink circulation path, and there is a high likelihood of bubbles merging with the ink through the circulation path, which is constituted by a tube or the like.
Furthermore, in the prior art the pump which circulates the ink and a pump which adjusts the internal pressure of the sub-tank are provided separately rather than being constituted by a single pump, making the apparatus constitution complicated and preventing size reductions.